Density phase separation effects

The ability of near-IR difiuse reflectance spectroscopy to characterise RIM PU elastomers was demonstrated. The specific effects of composition, density, phase separation, and moisture content on the near-IR spectra were identified. The ability of near-IR spectroscopy to make rapid assessments of physical properties is discussed. 52 refs. 

Phase densities differ by a Phase densities differ by only about 10%. factor of 100-10,000 1. Viscosity in both phases is Liquid phase viscosity moderate, solid low. phase rigid. Phase separation is rapid Phase separation is slow surface-tension and complete. effects prevent completion. Countercurrent contacting is Countercurrent contacting is slow and quick and efficient. imperfect. ... 

The need to separate solid and liquid phases is probably the most common phase separation requirement in the process industries, and a variety of techniques is used (Figure 10.9). Separation is effected by either the difference in density between the liquid and solids, using either gravity or centrifugal force, or, for filtration, depends on the particle size and shape. The most suitable technique to use will depend on the solids concentration and feed rate, as well as the size and nature of the solid particles. The range of application of various techniques and equipment, as a function of slurry concentration and particle size, is shown in Figure 10.10. 

PVA and TaM -for the 88%-hydrolyzed PVA. The same dependence was found for the adsorbed layer thickness measured by viscosity and photon correlation spectroscopy. Extension of the adsorption isotherms to higher concentrations gave a second rise in surface concentration, which was attributed to multilayer adsorption and incipient phase separation at the interface. The latex particle size had no effect on the adsorption density however, the thickness of the adsorbed layer increased with increasing particle size, which was attributed to changes in the configuration of the adsorbed polymer molecules. The electrolyte stability of the bare and PVA-covered particles showed that the bare particles coagulated in the primary minimum and the PVA-covered particles flocculated in the secondary minimum and the larger particles were less stable than the smaller particles. 

In most industrial applications, multistage countercurrent contacting is required. The hydrodynamic driving force necessary to induce countercurrent flow and subsequent phase separation may be derived from the differential effects of either gravity or centrifugal force on the two phases of different densities. Essentially there are two types of design by which effective multistage operation may be obtained ... 

Systematic studies of macrosyneresis in crosslinking dissolved polymer chains are lacking the available experimental data indicate, that eventual phase separation is a frequently occurring phenomenon 149, 11,2, 81). In crosslinking polymerization the effect of increasing cross-linking density is well established and always favors phase separation irrespective of the solvent power of the diluent (157). 

The reason for the experimentally proven lipophobicity, i.e., the tendency of fluorinated and hydrogenated chains to phase separate, is much less clear than the other effects of fluorination and is still under debate. Mostly it is assigned to the disparity of cohesive energy densities between perfluoroalkanes and alkanes. A reduction of ca. 10% in the interactions between unlike pairs of molecules was estimated by several methods [90]. However, there are also simulations suggesting slightly stronger attractive contributions to the interaction between Rf/Rh pairs compared to the like interactions under certain circumstances [94]. 9 However,... 

Efficient phase separation is critical, since cross-phase contamination has an inherently adverse effect on mass-transfer efficiency. In addition, carryover of solvent in aqueous effluent streams results in loss of solvent from the process, impacting process economics. Phase separation is affected by several physicochemical factors, including the viscosities and densities of the opposing bulk phases and the interfacial tension of the two-phase system. All of these properties contribute to the dimensionless dispersion number, which describes the tendency of two dispersed phases to separate... 

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